We are proposing to conduct a case-control study of the anal cancer to test the hypotheses that: 1. The practice of anal intercouse is related to an increased risk of anal cancer. 2. The mechanism through which anal intercouse produces this increased risk is the introduction of one (or more) sexually-transmitted viruses into anal tissue. The study will be conducted in three counties of Western Washington and in metropolitan Vancouver, British Columbia. Male and female residents who are diagnosed with anal cancer during 1/78-6/86 will be identified and interview regarding sexual practices and a history of sexually transmitted diseases. Blood samples will be collected for all study subjects and analyzed for evidence of prior exposure to sexually-transmitted viral infections. Tissue samples will be examined using in situ hybridization techniques to determine whether neoplastic cells in anal carcinoma contain any viral genes or gene products. On the basis of prior evidence the viruses most likely to be identified by these methods will be herpes simplex-2 and human papilloma virus.